First thing I'd do is run some really-good fuel-injection cleaner through it (Lucas). You may well have an injector hanging open due to dirt. It wouldn't be the first time I've seen a $4 bottle work a miracle.
Next thing I'd look at would be the coolant-temp sensor. If it's bad, the computer forever thinks the engine is cold, and runs it at a mixture like full choke on a carburetor... and that sensor rarely throws a code unless it goes full open. On some late-model Mopars the coolant-temp sensor also runs the gauge via the PCM, but there's a separate temp sender (for the dash gauge) available for your truck so the gauge won't indicate if this is the problem.
I'd also suspect the MAP sensor, though usually that will throw a code... I don't sell many MAP sensors for Chryslers of any ilk anymore much less trucks (during the 2.2L turbo era, I sold plenty) but it's a possibility.
I would think if it were a vacuum leak, it'd be running lean rather than rich. :huh: Still, if the IAC valve is stuck shut, you're essentially getting no air at idle, which would cause it to run fat--but only at idle, since once the throttle is opened the IAC is out of the equation. If it's running rich at cruise, under acceleration, etc., the IAC may be part of the problem, but it's not the problem.
My guess is this:
One common problem for which the V6/V8 Magnums are known is the plenum gasket, which is unfortunately under the intake. It can run rich because not only is it getting fuel, it's burning oil vapor from the engine's valley, even if you're not getting smoke.
If you look down the throttle body and see black, it's shot and needs to be changed. There's no way around it, and it's a tedious job. Here's a little hint if you need to change it: follow the gasket-change procedure to the letter. Do not try to skip a step (like torquing the bolts to final spec on the first try) and do not try to improve the gasket's seal by using a sealant. Do exactly what you're told, or you're going to be doing it again very soon. Halfway through the job, you won't ever want to pull that intake again. It's not that difficult, it's just very time-consuming to do it right.
I've never done one, but I know several people who have. The first was Stretch, and if memory serves, he had to do it twice on his own truck because the first time he didn't follow the instructions exactly. He's the one that stressed to me the importance of torquing in the correct order and in the correct steps as well as not attempting to use sealant because he knows I sell these gaskets frequently. I've had several DIY guys not listen to what I was saying and do the job twice (one guy three times) usually within a week or two. The guy that did it three times followed the torquing procedure to the letter on the second try, but still decided sealant would help. A week later he was back for another gasket. On this task, thus far, Stretch has never been wrong in five years.
Just the two cents of a parts guy... we hear stuff like this every day, and usually customers will come back and tell us their solution.
Have a peek down the throttle body, and I think you'll find your issue.
